Gravity-induced reorientation of cortical microtubules observed in vivo

Cortical microtubules play an important role during morphogenesis by determ ining the direction of cellulose deposition. Although many triggers are kno wn that can induce the reorientation of cortical plant microtubules, the re orientation mechanism has remained obscure. In our approach, we used gravit ropic stimulation which is a strong trigger for microtubule reorientation i n epidermal cells of maize coleoptiles. To visualize the gravitropically in duced microtubule reorientation in living cells, we injected rhodamine-conj ugated tubulin into epidermal cells of intact maize coleoptiles that were e xposed to gravitropic stimulation. From these in vivo observations, we prop ose a reorientation mechanism consisting of four different stages: (1) a tr ansitional stage with randomly organized microtubules; (2) emergence of a f ew microtubules in a slightly oblique orientation; (3) coalignment: neighbo uring microtubules adopt the oblique orientation resulting in parallel orga nized microtubules; and (4) the angle of these parallel, organized microtub ules increases gradually. Thus, the overall reorientation process could inc lude selective stabilization/ disassembly of microtubules (stage 2) as well as movement of individual microtubules (stages 3 and 4).